Age-related changes in many physiological parameters have long been known to occur. The basis for these alterations is, however, not well understood. Response to various stresses seems to decline with age. Changes in the ability to metabolize exogenous as well as endogenous compounds has been suggested as a cause of altered functions. This work will explore senescent changes in metabolism of several tissues--liver, lung, kidney, and small intestine tissues. Altered distribution and excretion of chemicals in aging animals is being studied in order to elucidate the basis for age-related changes in toxicological responses. Age-related alterations in gastrointestinal absorption are also being studied.